X-Ray Emission from Accreting Stellar Objects

Accreting stellar objects encompass a diverse population of white dwarfs, neutron stars and stellar-mass black holes that draw matter from a companion star or a surrounding disc. As gas spirals inward it forms a hot, dense accretion disc whose innermost regions reach temperatures of millions of kelvin, emitting predominantly in X-rays. In systems accreting below the Eddington limit, the disc and a tenuous, hot corona produce characteristic two-component spectra, whereas super-Eddington flows drive powerful radiatively driven winds and form geometries in which a funnel of emission shapes observer-dependent spectra. Pulsars introduce periodic modulations and cyclotron resonant scattering features that probe magnetic field strengths, while polarimetric measurements reveal the geometry and temperature structure of emission regions. Ultralluminous X-ray sources (ULXs) are a class of super-Eddington accretors whose luminosities exceed 10^39 erg s^–1 and whose spectral and timing properties bridge stellar-mass binaries and intermediate-mass black hole candidates. Together, spectroscopy, timing, polarimetry and high-resolution grating studies provide a holistic view of the physics of accretion, magnetohydrodynamics and radiation under extreme gravity, with implications for black hole growth, equation-of-state constraints for dense matter and the role of feedback in galactic environments.

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X-Ray Emission from Accreting Stellar Objects publication trend

The graph below shows the total number of articles in x-ray emission from accreting stellar objects across all publications each year (not limited to Nature Index journals).

Technical terms

Accretion disc: A rotating, flattened structure of infalling gas around a compact object, heated by viscous dissipation and radiating primarily in X-rays at small radii.

Super-Eddington accretion: A regime in which the mass accretion rate onto a compact object exceeds the value that produces the Eddington luminosity, driving strong radiative outflows.

Comptonised corona: A region of hot, optically thin or moderately thick plasma above the accretion disc that upscatters thermal photons to higher energies via inverse Compton scattering.

Cyclotron resonant scattering feature (CRSF): An absorption or emission line in the hard X-ray spectrum caused by electron transitions between quantised Landau levels in a strong magnetic field.

Polarimetry: The measurement of the orientation and degree of polarisation of X-ray photons, providing constraints on the geometry and magnetic field configuration of the emission region.

References

1. Ultraluminous X-ray sources. New Astronomy Reviews (2023).
2. IXPE Observations of the Quintessential Wind-accreting X-Ray Pulsar Vela X-1. The Astrophysical Journal Letters (2023).
3. XMM–Newton observations of the brightest ultraluminous X-ray sources. Monthly Notices of the Royal Astronomical Society (2006).
4. From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link. Monthly Notices of the Royal Astronomical Society (2017).